Form 4
Ratios and Percentages
Chapter 6 Ratios and Percentages
Section 6.1 Ratios Introduction
Ratios are used to compare quantities.
Ratios are written with a colon (:).
A ratio can be expressed in a number of ways. For example if John is five years old and Maria is 20 years
old, the ratio of their ages is:
John’s Age : Maria’s Age
which is:
5 : 20
which we can simplify to:
1 : 4 (dividing both sides by 5)
A ratio is usually given in one of these three ways.
John’s Age : Maria’s Age
or
5 : 20
or
1:4
John’s Age to Maria’s Age
or
5 to 20
or
1 to 4
or
or
Example 1
If there are 7 boys and 9 girls in a class the ratio of the number of boys to the number of girls is :
Boys : Girls
7:9
The order is very important since the ratio 9 : 7 shows girls : boys
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Form 4
Ratios and Percentages
Ratios can be simplified just like fractions.
The ratio 7 : 9 cannot be simplified because it does not have any common factors.
If there are 6 boys and 9 girls… the ratio of boys to girls would be
6:9
This can be simplified because 3 can be divided exactly into both 6 and 9.
2:3
This means that for every 2 boys in class there are 3 girls.
The ratio 2 : 3 is now in its simplest form since 2 and 3 have NO common factors.
Section 6.2 Simplifying ratios
Method 1: Normal Simplification just like fractions
150 : 30
15 : 3
(divide by 10 on both sides)
5:1
(divide by 3 on both sides)
Method 2: Common Units
When working with ratios involving different units, always convert them to the same units. A ratio can
be simplified only when the units of each quantity are the same, because the ratio itself does not have
any units. Once the units are the same we can simplify the ratios normally.
The ratio of 125 g to 2 kg must be converted to the ratio of 125 g to 2000 g so that you can simplify it.
125 : 2000
5 : 80
(divide both sides by 25)
1 : 16
(divide both sides by 5)
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Form 4
Ratios and Percentages
Method 3: Ratios as Fractions
We must change both fractions to whole numbers in order to be able to simplify. This can be
done by multiplying both sides of the ratio with the LCM.
LCM = 15
Method 4: Ratios as Decimal
0.8 : 1.2
In order to simplify decimals we must multiply by 10, 100 or 1000 in order to remove the
decimal point from both sides of the ratio.
0.8 : 1.2
x 10
8 : 12
4
2:3
Consolidation
Simplify the following ratios:
1. 42 : 70
2. 18 : 72
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Ratios and Percentages
3. 3.5 : 1.5
4.
2 ½ : 3 1/3
5. 5 1/3 : 4
6. 5 5/8 : 3 ¾
7. 3 kg : 250 g
8. 400m : 2 km
9. 1 hour : 1 day
10. 75 cents : $2
Support Exercise Pg 318 Exercise 20 A Nos 1 – 10
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Form 4
Ratios and Percentages
Section 6.3 Sharing a Ratio
To divide an amount in a given ratio, you first look at the ratio to see how many parts there are
altogether.
For example, 4: 3 has 4 parts and 3 parts giving 7 parts altogether.
7 parts is the whole amount.
1 part can then be found by dividing the whole amount by 7.
3 parts and 4 parts can then be worked out from 1 part.
Example
Divide $28 in the ratio 4 : 3
4 + 3 = 7 parts altogether
So 7 parts = $28
Dividing by 7:
1 part = $4
4 parts = 4 ╳ $4 = $16
3 parts = 3 ╳ $4 = $12
So $28 divided in the ratio 4 : 3 = $16 : $ 12
Consolidation
1. 400g in the ratio 2 : 3
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Ratios and Percentages
2. $100 in the ratio 2 : 3 : 5
3. A bag contains blue and red beads. Their ratio is 4 : 1. There are a total of 40 beads in the bag.
How many are blue and how many are red?
4. Maria and Amanda receive a total $35 as pocket money in the ratio of 4 : 3. Find
how much each girl receives.
5. The ratio of female to male members in a sports club is 7 : 3. The total number of members of
the group is 250.
a. How many members are female?
b. What percentage of members are male?
Support Exercise Pg 321 Ex 20C No 1 – 12
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Form 4
Ratios and Percentages
Section 6.4 Finding missing values with ratios
Example 1
A fruit drink is made by mixing orange squash with water in the ratio 2 : 3. How much water needs to
be added to 5 litres of orange squash to make the drink?
2 parts = 5 litres
1 part = 5  2 = 2.5 litres
3 parts = 2.5 ╳ 3 = 7.5 litres
So, 7.5 litres of water is needed to make the drink.
Example 2
Two business partners, Libby and Adam, divided their total profit in the ratio 3 : 5. Libby received
$2100. How much did Adam receive?
3 parts = $2100
1 part = 2100  3 = $700
5 parts = 700 ╳ 5 = $3500
So, Adam received $3500 of profits.
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Ratios and Percentages
Consolidation
1.
A blend of tea is made by mixing Lapsang with Assam in the ratio 3 : 5. I have a lot of tea but
only 600g of Lapsang. How much Assam do I need to make the blend using all the Lapsang?
2. The ratio of male to female spectators at a hockey game is 4 : 5. 4500 men watched the match.
What was the total attendance at the game?
3. Three business partners, Ren, Shila and Fatima, put money into a business in the ratio 3 : 4 : 5.
They shared any profits in the same ratio. Last year, Fatima made $3400 out of the profits. How
much did Ren and Shila make last year.
4. A ‘good’ children’s book has pictures in the ratio 17 : 8. In a book I have just looked at, the
pictures occupy 23 pages. How many pages of text should this book have?
Support Exercise Pg 319 Ex 20B
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Form 4
Ratios and Percentages
Section 6.5 Direct Proportion
If 1 pencil costs 15 cents, then
2 pencils cost 30 cents (2 x 15)
3 pencils cost 45 cents (3 x 15)
4 pencils cost 60 cents (4 x 15)
The cost depends on the number of pencils. As the number of pencils increases, the cost increases.
The cost is said to increase in the same proportion as the number of pencils increase the cost increases
proportionally.
The two quantities of the cost and price are said to be Directly Proportional.
Real Life Situations

The amount of petrol bought is directly proportional to the size of the petrol tank

The number of Euros exchanged to Dollars is directly proportional to the number of Euros
Example 1
5 buns cost $1.50. Work out the cost of 7 of these buns.
Find the cost of 1 bun
5 buns = 150 cents
1 bun = 150  5 = 30 cents
7 buns = 30 cents x 7 = 210 cents
7 buns = $2.10
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Ratios and Percentages
Example 2
3 pencils cost 96 cents. Work out the cost of 5 of these pencils
3 pencils = 96 cents
1 pencil = 96  3 = 32 cents
5 pencils = 32 x 5 = 160 cents
Consolidation
1. If 30 matches have a mass of 45g, what would be the mass of 40 matches?
2. Eight men can chop down 18 trees in a day. How many trees can 20 men chop down in a day?
3. Seventy maths textbooks cost $875
a. How much will 25 maths textbooks cost?
b. How many maths textbooks can you buy for $100?
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Ratios and Percentages
4. Find the cost of 48 eggs when 15 eggs can be bought for $2.10.
5. Five bars of chocolate cost $2.90. Find the cost of 9 bars.
Support Exercise Pg 323 Exercise 20D No 1 – 18
Section 6.6 Indirect Proportion
Inversely Proportional
A car travelling at a steady speed of 50 km/hr travels 200km in 4 hours.
A car travelling at a steady speed of 100 km/hr travels 200km in 2 hours.
As the speed increases, the time decreases.
As the speed is multiplied by 2, the time is divided by 2.
Two quantities are said to be in Inverse Proportion if one quantity increases at the same rate as the
other quantity decreases.
The product of the speed at the time gives the same number.
50 x 4 = 200
100 x 2 = 200
When two quantities are inversely proportional their product is constant.
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Ratios and Percentages
Example 1
A library has funds to buy 280 books that cost $15 each. How many books costing $10.50 could
be bought instead?
$15 = 280 books
$10.50 = ?
Example 2
When 9 hikers share out their packs of sandwiches equally they get 5 each. If 3 of the hikers did
not want any, how many would each of the others have?
9 hikers = 5 sandwiches
6 hikers = ?
Consolidation
1. Leo has some books which are 2 cm wide and he can fit 40 on a shelf. How many books 5 cm
wide could fit on the same shelf?
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Ratios and Percentages
2. A car travelling at 30 km/h takes 40 minutes to complete a journey. How long would it take
travelling at 60 km/h?
3. Chairs are available at three prices: $30, $40 and $60. Leah can afford to buy 24 and $60 each.
a. How many can she afford to buy at $40 each?
b. How many can she afford to buy at $30 each?
4. Some friends are sharing a taxi. If there are three friends it will cost $18 each. How much would
it cost if there were 5 friends?
Support Exercise Pg 325 Ex 20E No 1 – 12
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Form 4
Ratios and Percentages
Section 6.7 Introduction to Percentages
‘Per Cent’ means out of 100
From percent to fraction
1) 20% to a fraction
Write 20% as a fraction (20% means 20 out of 100)
20% =
2) 7% to a fraction
7% =
3) 35% to a fraction
35% =
4) 75% to a fraction
Percentage of a Quantity
1) 20% of 60
2) 60% of 350
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Form 4
3)
Ratios and Percentages
If 5% of Australians play tennis, how many people would you expect to play
tennis out of a group of 320
people?
Consolidation
Percentage
Decimal
Fraction
34%
0.85
3
/40
45%
0.3
2
/3
84%
0.45
3
/8
1. Calculate the following:
a. 15% of $300
b.
12% of 9 hours
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Form 4
Ratios and Percentages
c. 12.5% of $26
d.
16.2% of 265m
2. An estate agent charges 2% commission on every house he sells. How much commission will he
earn on a house that he sells for $120 500?
3. It is thought that about 20% of fans at a soccer match are women. For one match there were
42600 fans. How many of these do you think were women?
Support Exercise Pg 176 Ex 12A
Section 6.8 Increase and Decrease of Percentages
A Multiplier is a single number that an amount is multiplied by in order to increase or decrease
that amount.
Original
Amount
× multiplier
Final
Amount
Increase in Percentages
Example 1
Write the multiplier for a percentage increase of:
a) 64% =100% + 64% = 164% = 164/100 = 1.64
b) 21% = 100% + 21% = 121% = 121/100 = 1.21
c) 2 ½ % = 2.5% = ___________________________
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Ratios and Percentages
Example 2
Cedric, who was on a salary of $27 500, was given a pay rise of 7%. What is his new salary?
100% + 7% = 107%
107% = 1.07
1.07 ╳ $27 500 = $29 604
Decrease in percentages
Example 3
If there is a 10% discount off a shirt which has an original price of £20. What is the new price?
100% - 10% = 90%
90% = 0.9
20 ╳ 0.9 = €18
Example 4
A car valued at $6500 last year is now worth 15% less. What is the value now?
100% - 15% = 85%
85% = 0.85
0.85 ╳ $ 6500 = $5525
Make sure you identify whether he is asking you for an increase or decrease in
quantity.
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Ratios and Percentages
Consolidation
1. In 2005 the population of a city was 1 565 000. By 2011 it had an increase by 8%. What was the
population of the city in 2010?
2. A large factory employed 640 people. It had to streamline the workforce and lose 30% of the
workers. How big is the workforce now?
3. A bank pays 7% interest on the money that each saver keeps in the bank for a year. Allison
keeps $385 in the bank for a year. How much will she have in the bank after the year?
4. A television costs $400 before sales tax at 17.5% is added. How much will the television cost
after the tax is added?
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Form 4
Ratios and Percentages
5. A new diet guarantees that you will lose 12% of your mass in the first month. What mass should
the following people have after one month of diet?
a. Greta, who started at 52kg
b.
Pierre, who started at 75kg
Support Exercise Pg 179 Ex 12B
Section 6.9 A quantity as a Percentage of another, Profit and Loss
Example 1
What percent is $100 of $320?
Express $100 in $320 as a fraction
Therefore to get a percentage
Example 2
John obtained 60 marks out of 75 in a Mathematics test. Find the percentage of marks he got.
Fraction :
Percent:
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Ratios and Percentages
Example 3
What percent of $5 is 28 cents?
$5 = 500 cents
Fraction :
Percent:
Profit and Loss
Example 4
Alan bought a bowling ball for $12 and sold it for $15. What was his percentage profit?
Profit: $15 - $12 = $3
Percent:
Example 5
Zack bought a car for $20 000 and sold it for $18 000. What is his percentage loss?
Loss: $20 000 - $18 000 = $2 000
Percent:
Consolidation
1. What percentage is $5 of $20?
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Ratios and Percentages
2. What percentage is 105g of 1kg?
3. In Greece, there are 3 654 000 acres of agricultural land. Olives are grown on 237 000 acres of
this land. What percentage of agricultural land is used for olives?
4. Find the percentage profit on the following. Give your answers to 1 decimal place.
Item
Retail price (selling price)
Wholesale (price shop paid)
a. CD player
$89.50
$60
b. TV set
$345.50
$210
c. Computer
$829.50
$750
Support Exercise Pg 182 Exercise 12C
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Form 4
Ratios and Percentages
Section 6.10 Reverse Percentages
To find a percentage of a quantity we used to follow the procedure of
Original
Amount
× multiplier
Final
Amount
Now the final amount will be given to us and we have to find the original amount.
Original
Amount
÷ multiplier
Final
Amount
Example 1
In a sale the jackets are reduced by 40%
The sale price of a jacket is € 24
Work out the original price of the jacket.
100% - 40% = 60%
60% = 0.6
24  0.6 = €40
Example 2
Franco receives a 15% discount on a TV, and he pays $510. What was the price before the discount?
100 % - 15% = 85%
85% = 0.85
510  0.85 = €600
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Ratios and Percentages
Example 3
Melissa buys blouses for her clothes store. She marks the price up by 60%. If she sells a blouse for $74.25,
how much did the blouse cost her?
100% + 60% = 160%
160% = 1.6
74.25  1.6 = €46.41
Consolidation
1. Tina’s weekly pay is increased by 5% to $315. What was Tina’s pay before the increase?
2. The number of workers in a factory fell by 5% to 228. How many workers were there originally?
3. Mario buys a car and sells it for @2940. He made a profit of 20%. What was the cost price of the
car?
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Ratios and Percentages
4. After an 8% increase, the monthly salary of a chef was $1431. What was the original monthly
salary?
5. Oliver buys a chair. He sells it for $63 in an auction and makes a loss of 55%. What did he pay for
the chair?
Support Exercise Pg 188 Ex 12E
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